I'll be writing a letter to tell them about my grades, but it's only to Northwestern, and it's going to be a LOLOI - letter of lots of interest! I wouldn't bother updating schools that haven't given you an interview, unless you really want to go there, in which case, you should write a letter, and maybe attach an informal transcript. They'll trust you - if you lied about your grades, they'll find out when they get your final transcript, and I'm sure you'd be expelled.